Plasma arc torches frequently employ a shield in combination with a nozzle to direct a shield gas onto an ionized plasma stream flowing from a plasma torch. Some of these shields have been configured to direct the shield gas normal to the path of the ionized plasma, which is felt to provide enhanced cooling and protection of the nozzle from slag, while others direct the shield gas to move substantially parallel to the ionized plasma gas, which is felt to enhance the stability of the plasma arc to improve the quality of the cut and avoid undue wear on the electrode of the torch caused by erosion. An alternative approach, used by ESAB AB in torches such as its PT-19™ model, is to direct the shield gas toward the plasma arc at an angle that intersects the arc above the work-piece, to provide a balance between the benefit of cooling and protection of the nozzle, and the benefit of stability of the resulting arc. These approaches are all discussed in U.S. Pat. No. 8,395,077, which teaches a preferred range of geometries for a shield and nozzle combination which direct the gas at an angle.
FIG. 1 is a stylized section view showing a portion of a prior art plasma arc torch 10 that directs the shield gas at an angle, such as taught in the '077 patent. The torch has a nozzle 12 having a distal end region 14 with a conical exterior surface 16, where the cone is defined by a prescribed range of half angle α of the cone with respect to a nozzle axis 18. A matched shield 20 has a conical interior surface 22 with a similar half angle β. The combination of the conical exterior surface 16 of the distal end region 14 and the conical inner surface 22 of the shield 20 serve to form an angled annular passage 24 to direct the shield gas toward the ionized plasma at an angle γ (determined by the angles α and β of the nozzle and shield surfaces) with respect to the nozzle axis 18. The conical exterior surface 16 terminates at a distal end face 26 of the nozzle 12, this distal end face 26 circumscribing a nozzle orifice 28 and having an end face diameter Φ1. The nozzle orifice 28 has a hydraulic diameter D, and the '077 patent includes preferred ratios of Φ1:D in the various parameters that are intended to provide enhanced performance. The end face diameter Φ1 and the angle γ of the shield gas result in the gas intersecting the plasma arc at a merge point M.